1. Field of the Invention
The present invention relates, generally, to methods for forming dies by chemical milling and the resulting dies and, more especially, to methods for improving the process-ability of steel dies destined to be circuit stamping dies used to fabricate circuit boards. The present invention further relates to an improved stamping die exhibiting enhanced lubricity and/or wear resistance with reduced tendencies toward oxidation during use, inter alia, in the fabrication of circuit boards.
2. Description of the Background Art and Technical Problems
The present inventor's U.S. Pat. No. 3,758,350, entitled "Method of Making a Die for Stamping Out Circuit Boards", discloses a method for etching a die blank to form an improved die particularly adapted for manufacturing a relatively compact circuit board where the circuit elements are of relatively small width and with small separation therebetween. In that method, first there is formed a conventional flat face die, comprising flat face die elements with intervening recess areas, this being accomplished in a conventional manner. Then the recess areas are filled with an etchant-resistant filler such as epoxy, and a photo resist is applied over the filler in a manner to overlap the outer edge portions of the die elements so that the middle portion of each die element is etchant active. The die is then exposed to an etching medium, such as ferric chloride, to form a die element recess in the middle portion of each die element with shoulders along the edge portions thereof. Next the resist material is removed and, with the filler still remaining in the recess, the die surface is again etched for a short period of time. In one procedure the final etching is done in a manner to provide "modified" die element edge portions where the inner edge surface of each die element edge portion is somewhat rounded, and in another procedure the final etching is accomplished in a manner to provide a knife edge die.
The methods disclosed in the aforementioned patent have proven quite effective in making precision dies for the stamping of circuit boards and the trimming of circuits borne on flexible substrates. Nonetheless, certain improvements over those basic techniques have been made by the present inventor, particularly relating to the manufacture of dies for stamping circuit boards from heavier foil. It has now become somewhat commonplace to use thicker foil elements in the manufacture of circuit boards to achieve, for example, the ability of the board to carry higher current loads. In turn, this has somewhat complicated the design of suitable dies for that purpose.
In a circuit board manufacturing operation it is desirable to accomplish both the forming of the circuit elements from the foil sheet and the bonding of these circuit elements to a dielectric substrate in one stamping operation. This is usually accomplished by providing a thermo-adhesive coating between the foil sheet and the substrate and heating the die prior to the stamping operation. When the heated die is brought into engagement with the foil sheet, the die elements press into the foil sheet to separate the circuit elements therefrom and press these elements into firm engagement with the substrate while heating the underlying adhesive layer to cause bonding of the circuit elements thereto. The excess foil adjacent the die elements does not become bonded to the substrate, and this excess foil is simply stripped away from the dielectric material after the stamping sequence.
When it is attempted to use heavier foils to make circuit boards where the width and spacing of the elements are quite small, the problems of accomplishing this simultaneous forming and bonding of the circuit elements properly are greatly aggravated. First, with regard to the individual die elements, the edge portions of each element must be of a height sufficient to cause a separation through the entire thickness of the foil sheet. Second, the depth of the recess areas of the die must be sufficient to permit the excess foil to become positioned in the recesses during the stamping operation, without the excess foil becoming bonded to the substrate. Thus, the recessed areas of the die must have a depth sufficient to accommodate the excess foil, while the die elements must have and retain a sharp cutting profile lest a dull or rounded edge (due perhaps even to oxidation of the die) leaves a small ribbon of foil which can bridge the circuit elements to yield an undesired electrical connection. These same considerations are equally applicable to trimming dies used to trim circuits on a flexible substrate. Accordingly, it is important that the raised die elements be formed precisely to avoid this unacceptable result. It is also important to the precision of any subsequent sharpening procedure that the raised stamping element be formed precisely.
Although the goal of forming raised die elements in a very precise pattern and within carefully controlled tolerances is easily articulated, realization of that goal is sometimes quite elusive. Notwithstanding the high technological development in the art, it too often occurs that the raised elements in the die are imperfectly formed regardless of the care taken to prepare and then process the die blank. Surprisingly, a die blank believed to be of high quality steel and prepared with care will sometimes be found to have raised die elements inexplicably out of tolerance or having rounded edges resulting in the need either to scrap the die and begin again or to undertake tedious, labor-intensive efforts to salvage that die. Considerable effort and capital resources have been expanded to detect, if not understand, how and why seemingly properly prepared die blanks surprisingly run out of tolerance during intermediate fabrication techniques.
Yet a further consideration in the subsequent use of these types of dies, irrespective of the source of fabrication difficulties in achieving acceptable die conformations, is the ease with which the die elements may slip through and sever the foil in a stamping operation; a factor of particular significance in the case of stamping circuits from thicker foils as noted in general above. Factors which influence this aspect of operation include lubricity of the die, somewhat related wear resistance and any surface oxidation. Increased die lubricity, as one would reasonably expect, improves the ease of cutting the foil and severing the same while tending to minimize wear along sharp knife edges bounding the circuit side of the die from the cavity side of the die. Wear resistance of the sharp cutting edges, somewhat apart from die lubricity, has a demonstrable effect on die longevity, reducing the frequency of periodic resharpening to maintain those edges. Likewise, as steel dies have a tendency to oxidize during periods of operational interruption, the presence of oxide along the knife edge regions reduces stamping efficiency and can, in many circumstances, give rise to unacceptable product. A goal along these lines is an improvement in the lubricity of a die with reduced oxidation tendencies, but without the introduction of an ancillary or foreign composition (e.g., a hydrocarbon lubricant or the like) which could interfere with the circuit stamping process, while contributing to improved wear resistance.